RFR: 8295944: Move the Http2TestServer and related classes into a package of its own [v3]

[Jaikiran Pai]

> Can I please get a review of this change which addresses https://bugs.openjdk.org/browse/JDK-8295944?
> 
> As noted in the issue, the HttpClient testing helper infrastructure code resides in a default (unnamed) packages within the test hierarchy. The commit in this PR moves those classes into a specific package. The tests have been updated to refer to them correctly with proper package name references.
> Additionally as noted in the issue, the `@modules` declaration used in the httpclient tests, were all the same and were repeated in all these tests. The commit in this  PR moves those module references into the `TEST.properties` to centralize such reference. Any test which needs additional modules can then use specific `@modules` declaration in the test definition.
> 
> No functional changes have been done in this PR.
> 
> tier1, tier2 and tier3 testing passed after these changes.

Jaikiran Pai has updated the pull request incrementally with two additional commits since the last revision:

 - be consistent and use the full path of the library
 - remove left over content from the @modules that were removed

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/12160/files
  - new: https://git.openjdk.org/jdk/pull/12160/files/97cbdc2e..ec0a6889

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=12160&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=12160&range=01-02

  Stats: 40 lines in 22 files changed: 0 ins; 39 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/12160.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12160/head:pull/12160

PR: https://git.openjdk.org/jdk/pull/12160